Magnetic random access memory with synthetic antiferromagnetic storage layers

ABSTRACT

A synthetic antiferromagnetic device includes a first tantalum layer, a reference layer disposed on the first tantalum layer and including a first cobalt iron boron layer, a second cobalt iron boron layer disposed on the first cobalt iron boron layer, a third cobalt iron boron layer and a second tantalum layer disposed between the second and third cobalt iron boron layers, a magnesium oxide spacer layer disposed on the reference layer and a cap layer disposed on the magnesium oxide spacer layer.

BACKGROUND

The present invention relates to magnetic random access memory devices,and more specifically, to magnetic random access memory with a syntheticantiferromagnetic (SAF) freelayer with a non-exchange-coupled spacerlayer.

SAF storage layers are implemented in magnetic random access memorydevices (MRAM) with certain advantages. For example, it appears thathigher activation energy can be obtained for equivalent switchingcurrents when using a SAF structure versus a simple single free layer.Currently, the results in the literature that demonstrate such advantageare obtained with low-temperature annealing and processing. However, formany semiconductor applications (particularly for embedded memoryapplication) it is important that the process be compatible withexisting CMOS fabrication requirements. In practice, this means that theMRAM device must be able to withstand high temperature annealing andprocessing, with temperatures in the range of 400° C. for 1 hour totalexposure time. Currently, it is also possible to fabricate SAFstructures, which survive 400° C. anneals, demonstrating strong couplingin the SAF. Spin torque switching has been observed for annealtemperatures of up to 350° C. However, the current density can be toolarge to be practically employed, and even for these demonstrations theactivation energy reported was only 62 kT. For applications in MRAM, anactivation energy of at least 80 kT is typically required in order toretain information for sufficiently long times.

SUMMARY

Exemplary embodiments include synthetic antiferromagnetic device,including a first tantalum layer, a reference layer disposed on thefirst tantalum layer and including a first cobalt iron boron layer, asecond cobalt iron boron layer disposed on the first cobalt iron boronlayer, a third cobalt iron boron layer and a second tantalum layerdisposed between the second and third cobalt iron boron layers, amagnesium oxide spacer layer disposed on the reference layer and a caplayer disposed on the magnesium oxide spacer layer.

Additional exemplary embodiments include a synthetic antiferromagneticdevice including a reference layer, a spacer layer disposed on thereference layer, a storage layer disposed on the spacer layer and a caplayer disposed on the storage layer, wherein at least one of thereference layer and the storage layer include a refractory metal layerdisposed between two layers of cobalt iron boron.

Further exemplary embodiments include a synthetic antiferromagneticdevice, including a substrate, a first tantalum layer disposed on thesubstrate, a first cobalt iron boron layer disposed on the firsttantalum layer, a second cobalt iron boron layer disposed on the firstcobalt iron boron layer, a second tantalum layer disposed on the secondcobalt iron boron layer, a third cobalt iron boron layer disposed on thesecond tantalum layer, a magnesium oxide spacer layer disposed the thirdcobalt iron boron layer, a fourth cobalt iron boron layer disposed onthe magnesium oxide spacer layer, a third tantalum layer disposed on thefourth cobalt iron boron layer, a fifth cobalt iron boron layer disposedon the third tantalum layer, a cap layer disposed on the fifth cobaltiron boron layer, a hard mask tantalum nitride layer disposed on the caplayer, a bottom contact coupled to the substrate and a top contactcoupled to the hard mask tantalum nitride layer, wherein the fourthcobalt iron boron layer, the third tantalum layer and the fifth cobaltiron boron layer; form a storage layer.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates an exemplary a SAF device;

FIG. 2 illustrates exemplary SAF devices in further detail; and

FIG. 3 illustrates a SAF fabrication method in accordance with exemplaryembodiments.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary a SAF device 100 that includes highanneal temperature SAF storage (free) layers for MRAM devices. Thedevice 100 illustrates two SAF devices 105, although it will beappreciated that additional or fewer SAF devices are contemplated inother exemplary embodiments. The device 100 includes a top contact 110and a bottom contact 115 coupled to the SAF devices 105.

In exemplary embodiments, each of the SAF devices 105 includes areference (pinned or non-pinned) layer 120 coupled to the bottom contact115. In exemplary embodiments, the reference layer 120 can be anystandard reference layer to provide a reference electrode. The SAFdevices 105 further include a spacer layer 125 disposed on the referencelayer 120 and a storage layer 130 disposed on the spacer 125. The SAFdevices 105 further include a cap layer 135 (which can include a hardmask) disposed between the storage layer 130 and the top contact 110.

In exemplary embodiments, the SAF devices 105 described hereinincorporate the storage layer 130, which combines both thermal stabilityat high anneal temperatures implemented in device fabrication for therequired period and switching with appropriately low switching currents,device resistance, activation energy and coercivity to allow properfunctioning in an MRAM device.

In exemplary embodiments, the systems and methods described hereincombine lower switching current with high temperature stability in SAFdevices. As such, MRAM devices include an SAF freelayer with anon-exchange-coupling spacer layer. The non-exchange-coupled SAF canimplement refractory materials and similar spacers and caps as furtherdescribed herein. In exemplary embodiments, an SAF device for an MRAMdevice includes a magnetic storage with a spacer made of Ta, TaN, W, orsimilar refractory (i.e. low diffusion) material. This structure can beplaced either below or above the tunneling barrier (e.g., MgO). The seedor cap layers can be one of the same materials used for the spacer, andthese can be mixed and matched (i.e. Ta seed, W spacer and the like). Inexemplary embodiments, the SAF devices described herein can include nopinning layer, but instead implement a double SAF device. The referencelayer SAF is identical in structure to the freelayer SAF, except thatthe magnetic films are thicker. In this way, the magnetic layersincluding the reference layer do not switch.

FIG. 2 illustrates the SAF devices 105 in further detail. FIG. 3illustrates a SAF fabrication method 300 in accordance with exemplaryembodiments. At block 305, the substrate 200 is prepared. The substrate200 can be silicon (Si). In order to attain magnetic tunnel junction(MTJ) performance in the final MRAM device, preparation of the substrate200 includes attaining atomic-scale flatness over areas of the order ofthe size of an MTJ. Atomic scale flatness reduces dipolar magneticcoupling effects and makes for a well-controlled MRAM device. Substratesfor the processes are thus generally prepared either with a carefulsilicon wafer oxidation/cleaning or with the deposition of a dielectricsuch as silicon nitride (SiN) on the substrate 200, followed by achemical mechanical planarization (CMP) step to smooth the surface.

At block 310, the reference layer 120 is deposited. As described herein,the reference layer 120 can include any standard reference layer, whichcan include a first tantalum (Ta) layer 205 to promote polycrystallinegrowth. In the example of FIG. 2, the first Ta layer 205 could betantalum nitride (TaN). In exemplary embodiments, depending on theimplementation, the reference layer 120 can then further include avariety of other layers including but not limited to an antiferromagnetfor strong pinning of the reference layer (e.g., PtMn or IrMn), and anantiferromagnetically exchange-biased pair of ferromagnets (e.g.,CoFe/Ru/CoFe). In the example of FIG. 2 a first cobalt iron boron(CoFeB) layer 210 is deposited on the seed layer 205, followed by asecond CoFeB layer 211. It will be appreciated that the first and secondCoFeB layers can be a single integrated CoFeB layer. A second. Ta layer215 is then deposited on the second CoFeB layer 211. Finally, a thirdCoFeB layer 212 is deposited on the second Ta layer 215.

At block 315, the spacer layer 125 is deposited on the reference layer120. In exemplary embodiments, the spacer layer 125 is a suitableinsulating electrical bridge (e.g., aluminum oxide (Al₂O₃) or magnesiumoxide (MgO)). In the example in FIG. 2, the spacer layer is an MgO layer220.

At block 320, the storage layer 130 is deposited on the spacer layer125. In the example in FIG. 2, a fourth CoFeB layer 230 is deposited onthe spacer layer 125, followed by a deposition of third Ta layer 235 anda fifth CoFeB layer 240.

At block 325, a suitable cap layer 135 is deposited on the storage layer130. In the example shown in FIG. 2, the cap layer 135 includes a fourthTa layer 245 deposited on the storage layer 130 followed by a depositionof a ruthenium (Ru) layer 250. In exemplary embodiments a hardmask TaNlayer 255 can be deposited on the Ru layer 250. The cap layer 135 andthe hardmask TaN layer protect the SAF device 105 during the aggressiveetch which defines the device 100

As described herein, the exemplary SAF devices 105 include a storagelayer 130 with a spacer made of Ta, TaN, W, or similar refractory (i.e.low diffusion) material. This structure can be placed either below orabove the spacer layer 125. FIG. 2 illustrates this structure both belowand above the spacer layer 125 respectively as the layers including thesecond CoFeB layer 211, the second Ta layer 215 and the third CoFeBlayer 212, and as the layers including the fourth CoFeB layer 230, thethird Ta layer 235 and the fifth CoFeB layer 240. In addition, the seedor cap layers can be one of the same materials used for the spacer inthe SAF storage layer 130, and these materials can be mixed and matched(i.e., Ta seed, W spacer and the like). As shown the first Ta layer 205is the seed layer and the cap layer 135 includes the fourth Ta layer245.

At block 330, further semiconductor processing can be implemented. Forexample, the SAF device 100 can be encapsulated in a dielectric. Theencapsulation of the SAF device 100 protects the SAF device 100 while atthe same time forming the environment in which the attachment of the topand bottom contacts 110, 115 can be implemented. The choice ofencapsulation is determined from three requirements: a) it must notdamage the SAF devices 100; b) it must adhere well to the substrate 200;and c) it should closely emulate the interlayer dielectrics (ILDs) thatwould be used in a fully integrated wafer processes. For example, damageto the MTJs can arise from chemical interactions and thermal stress.Standard semiconductor-industry dielectrics typically are deposited orcured at temperatures around 400° C., whereas degradation in submicronMTJs can set in at temperatures below 350° C. Thus, a major challenge tothe integrator of MRAM devices is the development and utilization ofsuitable low temperature dielectrics. However, it will be appreciatedthat the SAF device can withstand the high anneal temperatures discussedabove. Adhesion of the dielectric to the substrate can be particularlyproblematic given the characteristics of the magnetic films being used.Noble-metal-containing antiferromagnets can be particularly difficult toadhere to, and, if exposed by the etching used for MTJ patterning, canrequire specialized surface-cleaning or surface-preparation techniquesto promote adhesion to the encapsulating dielectric. The dielectricthickness is chosen such that it will be thick enough to provide theenvironment for the wiring level above the MTJs.

Other semiconductor processing includes suitable patterning of thespacer layer 125. Patterning of the MTJ introduces device-to-deviceisolation in the counter-electrode the conductive portion of the storagelayer 130 above the spacer layer 125), but maintains electricalcontinuity between desired devices in the base electrode (the conductiveportion of the reference layer 120 below the spacer layer 125). Oftennegligible in fully integrated wafers, the resistance of the baseelectrode after MTJ patterning is germane to the short loop. The use ofa continuous planar base electrode incurs additional measurement errorat final electrical testing if the base electrode possesses a high sheetresistance. Subject to the constraint of emulating the stack used infully functional wafers, the magnetic stack of the short loop willtherefore include thick or low resistivity films beneath the tunnelbarrier. A commonly used, straightforward approach to patterning theMTJs is through the use of a conducting hard mask. The conducting maskis later utilized as a self-aligned stud bridging the conductive MTwiring to the active magnetic films in the device. Such a processingscheme is among the simplest and fastest ways of creating and contactingthe MTJs, making it an ideal approach for use in the short loop. Choicesfor the hard mask are numerous, with necessary characteristics beingetchability and a resistance that is negligible when compared with MTJresistance. Refractory materials commonly used in the semiconductorindustry such as Ta, TaN, and TiN are suitable as masks for MTJpatterning. The MTJ shapes are defined in the hard mask by transfer froma first photomask level in a process such as the following: applyresist/expose and develop/reactive ion etching (RIE) through hardmask/strip resist. The pattern is further transferred downward topenetrate to (or through) the tunnel barrier, leaving behind alow-resistance base layer which covers the entire wafer.

Other semiconducting fabrication processes can include planarization. Tofacilitate industry standard damascene copper wiring, the wafersgenerally undergo a gentle dielectric CMP process at this stage. Thepurpose of the CMP is to remove topography from the surface that iscaused by the underlying MTJs. This step is also the first check of theadhesion of the dielectrics to the underlying metal films, as well asthe cohesion of the metal films to each other. If the encapsulatingdielectrics are suitably planarizing in their deposition, this CMPplanarization step can be eliminated for faster turnaround time andpotentially higher yield.

After completion of the steps for MRAM development (e.g., layerformation, patterning, and encapsulation), the wiring (e.g., applyingthe bottom and top contacts 110, 115) is instituted in the simplestmanner consistent with the available tooling. Relying onwell-established semiconductor-industry techniques, a photomask definedtrench is etched into the dielectric with RIE, to be filled with a linerand high-conductivity copper. The depth of the trench is sufficient toexpose a portion of the conducting hard-mask stud (thecounter-electrode), while not so deep as to create a short circuit tothe planar base electrode. Endpointing during the trench RIE canfacilitate the proper choice of trench depth even for relatively thinhard-mask films. After the trench etching and a suitable cleaning step,the wiring liner film is deposited, along with thin copper seed layer.This deposition is followed by the electroplating of copper tocompletely fill the trench and provide enough overburden so that theensuing CMP step will planarize the metal coincident with the surface ofthe dielectric. This final CMP step can be aggressive enough to causeshear failure of the films on the wafers, and care must be taken toprevent such delamination. A post-polish cleaning of the wafers is thefinal preparation step before electrical testing.

Example

In one embodiment, the best thermal stability after the differentcuring/annealing fabrication steps and combination of switching voltageand magnetoresistance, occurred for the composition as now described.The first Ta layer 205 is 20 angstroms (Å). The first CoFeB layer 210 is25 Å and the specific composition is Co₂₀Fe₆₀B₂₀. The second CoFeB layer211 is 10 Å and the specific composition is Co₂₀Fe₆₀B₂₀. The second Talayer 215 is 10 Å. The third CoFeB layer is 30 Å and the specificcomposition is Co₂₀Fe₆₀B₂₀. The spacer layer 125 of MgO is formed bydepositing 8 Å of Mg following by a 60 second exposure to oxygen gasunder a rate of 100 standard centimeter cube per minute (sccm). Anotherlayer of 3 Å of Mg is deposited followed by a 60 second exposure tooxygen under a rate of 50 sccm. Finally, another layer of 2 Å of Mg isdeposited. The storage layer 130 is then deposited with 15 Å of thefourth CoFeB layer 230. The third Ta layer 245 is then deposited at athickness of 10 Å. The fifth layer of CoFeB 240 is then deposited at athickness of 15 Å and a specific composition of Co₂₀Fe₆₀B₂₀. The caplayer 135 is then deposited including the fourth Ta layer 245 at athickness of 10 Å, and the Ru layer 250 at a thickness of 200 Å.Finally, the hard mask TaN layer 255 is deposited at a thickness of 1000Å.

It has been determined that the exemplary SAF devices (e.g., the SAFdevice 100) described herein combine lower switching current with hightemperature stability in SAF devices for implementation in MRAM devices.The non-exchange-coupled SAF can implement refractory materials andsimilar spacers and caps as further described herein. This structure canbe placed either below or above the tunneling barrier. The seed or caplayers can be one of the same materials used for the spacer, and thesecan be mixed and matched. In exemplary embodiments, the SAF devicesdescribed herein can include no pinning layer, but instead implement adouble SAF stack. The reference layer SAF is identical in structure tothe freelayer SAF, except that the magnetic films are thicker. In thisway, the reference layers don't switch. The SAF devices described hereinallow for improved thermal stability of the SAFs employed in the stack.Even though no exchange coupling is afforded by this SAF structure, theultimate performance is good, and improved thermal stability is asignificant improvement over conventional Ru-coupled SAF structures, forexample. Note that magnetically, the exemplary SAF devices describedherein behave much differently than the Ru-exchange-coupled SAFs (eitherAP or P orientation). However, experimental evidence suggests that theexemplary devices improve switching performance even if they do so in adifferent mode.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A synthetic antiferromagnetic (SAF) device,comprising: a first tantalum layer; a reference layer disposed on thefirst tantalum layer and including: a first cobalt iron boron (CoFeB)layer; a second CoFeB layer disposed on the first CoFeB layer; a thirdCoFeB layer; and a second Ta layer disposed between the second and thirdCoFeB layers; a magnesium oxide (MgO) spacer layer disposed on thereference layer; and a cap layer disposed on the MgO spacer layer. 2.The device as claimed in claim 1 further comprising a storage layerdisposed between the MgO spacer layer and the cap layer.
 3. The deviceas claimed in claim 2 wherein the storage layer comprises: a fourthCoFeB layer; a fifth CoFeB layer; and a third Ta layer disposed betweenthe fourth and fifth CoFeB layers.
 4. The device as claimed in claim 3wherein the cap layer comprises: a fourth Ta layer disposed on the fifthCoFeB layer; and a ruthenium (Ru) layer disposed on the fourth Ta layer.5. The device as claimed in claim 1 further comprising a substrate,wherein the first Ta layer is disposed on the substrate.
 6. The deviceclaimed in claim 2 wherein the MgO spacer layer is magnetically coupledto the storage layer, after the device is subject to anneal temperaturesof about 375° C. to 400° C.
 7. The device as claimed in claim 3 whereinthe first, second, third, fourth and fifth CoFeB layers are Co₂₀Fe₆₀B₂₀.8. A synthetic antiferromagnetic (SAF) device, comprising: a referencelayer; a spacer layer disposed on the reference layer; a storage layerdisposed on the spacer layer; and a cap layer disposed on the storagelayer, wherein at least one of the reference layer and the storage layerinclude a refractory metal layer disposed between two layers of cobaltiron boron CoFeB.
 9. The device as claimed in claim 8 wherein the caplayer comprises a ruthenium (Ru) layer disposed on a tantalum layer. 10.The device as claimed in claim 9 further comprising: a substrate; and aseed layer disposed on the substrate, wherein the reference layer isdisposed on the seed layer.
 11. The device as claimed in claim 8 whereinthe reference layer comprises: a first tantalum (Ta) layer; and a firstCoFeB layer disposed on the first Ta layer;
 12. The device as claimed inclaim 11 wherein when the refractory metal layer disposed between thetwo layers of CoFeB is disposed in the reference layer, the referencelayer further comprises: a second CoFeB layer disposed on the firstCoFeB layer; a third CoFeB layer; and a second Ta layer disposed betweenthe second and third CoFeB layers.
 13. The device as claimed in claim 11wherein when the refractory metal layer disposed between the two layersof CoFeB is disposed in the storage layer, the storage layer furthercomprises: a fourth CoFeB layer disposed on the spacer layer; a fifthCoFeB layer; and a third Ta layer disposed between the fourth and fifthCoFeB layers.
 14. The device as claimed in claim 8 further comprising ahard mask tantalum nitride (TaN) layer disposed on the cap layer. 15.The device as claimed in claim 8 wherein the refractory metal layer is atantalum (Ta) layer.
 16. The device claimed in claim 8 wherein thespacer layer is magnetically coupled to the storage layer, after thedevice is subject to anneal temperatures of about 375° C. to 400° C. 17.The device as claimed in claim 16 wherein two layers CoFeB areCo₂₀Fe₆₀B₂₀.
 18. A synthetic antiferromagnetic (SAF) device, comprising:a substrate; a first tantalum (Ta) layer disposed on the substrate; afirst cobalt iron boron (CoFeB) layer disposed on the first Ta layer; asecond CoFeB layer disposed on the first CoFeB layer; a second Ta layerdisposed on the second CoFeB layer; a third CoFeB layer disposed on thesecond Ta layer; a magnesium oxide (MgO) spacer layer disposed the thirdCoFeB layer; a fourth CoFeB layer disposed on the MgO spacer layer; athird Ta layer disposed on the fourth CoFeB layer; a fifth CoFeB layerdisposed on the third Ta layer; a cap layer disposed on the fifth CoFeBlayer; a hard mask tantalum nitride (TaN) layer disposed on the caplayer; a bottom contact coupled to the substrate; and a top contactcoupled to the hard mask TaN layer, wherein the fourth CoFeB layer, thethird Ta layer and the fifth CoFeB layer; form a storage layer.
 19. Thedevice claimed in claim 18 wherein the MgO spacer layer is magneticallycoupled to the storage layer, after the device is subject to annealtemperatures of about 375° C. to 400° C.
 20. The device as claimed inclaim 18 wherein the first, second, third, fourth and fifth CoFeB layersare Co₂₀Fe₆₀B₂₀.